US3317203A - Radiation shield for induction furnace - Google Patents

Description

- May 2, 1967 M. LlTZ ET AL 3,317,203 SHIELD I D CTION FURNACE United States Patent 3,317,203 RADIATION SHIELD FOR INDUCTION ACE and Larry M. Baker, Berea, Carbide Corporation, a cor- This invention relates to an improved radiation shield for use in induction heated systems. More particularly, it relates to a radiation shield having a plurality of layers, each layer containing multiple diagonal slits.

Induction furnaces for heating purposes are Well known in the art. Such furnaces generally consist of a crucible for holding the material to be melted and a closely positioned alternating current electric coil connected to an appropriate power supply. The crucible is constructed of electrically conductive material in which electrical currents are induced through coupling with the alternating electrical field generated by the alternating current coil. The crucible or susceptor is thereby heated to temperatures of or approaching white heat and thus heats the contained material by conduction. The susceptor crucible must be fabricated from refractory material, such as graphite, which has a melting point higher than that of the contained material to be melted. The susceptor crucible tends to transmit a considerable amount of heat to the surrounding areas by radiation since the crucible is heated to at least red heat and generally to temperatures of or approaching white heat. Many techniques have been employed in the prior art to minimize this radiation heat loss. One technique is to encase the susceptor in opaque thermal insulation such as thoria, zirconia and the like. Another technique is to employ various radiat-ion shields surrounding the susceptor. The prior art radiation shields have been bulky, complex in construction, expensive and have been limited in their useful maximum working temperature. This has been especially true in induction furnace-s employed in sintering high melting point metals.

It is an object of the present invention to provide a radiation shield for induction heated furnaces which is compact, simple of construction and is more effective than prior art radiation shields.

FIG. 1 is a side view of sheet material employed in this invention showing the multiple diagonal slits.

FIG. 2 is a top view of a preferred embodiment of the present invention.

FIG. 3 shows another form of the present invention.

The present invention comprises a radiation shield comprising a plurality of overlapping spaced layers wherein each layer contains multiple diagonal 'slits. Spacers are employed between adjacent layers to minimize electrical short circuits between layers and also to maximize radiation insulation efiiciency of the plural layers. In FIG. 1, the sheet material used for the improved radiation shield contains a plurality of diagonal slits 12. In order to maintain a substantially self supporting sheet 10, the slits 12 should not extend through the entire width of sheet 10. The slits should be spaced from about /2 to 1 /2 in. apart and preferably about 1 in. apart. The dimension A-A shown in FIG. 1 is genenally selected as the height of the susceptor around which the radiation shield is to be placed. The dimensions BB and C--C are selected so as to provide suflicient support strength to the sheet 10 and also to minimize the closed electrical circuit paths within a single sheet and thus reduce the induced currents in sheet 10 when it is employed in an alternating electrical field. Dimensions B-B and CC are conveniently about 1 in. In a given sheet 10 all the slits 12 are parallel to each other. Such spacer if the layers tend to move slits break up the closed electrical paths within the sheet and thus minimize any induced electrical currents. These slits are positioned diagonally across the width of sheet 10 in order to most effectively break up electrical currents which prefer to be induced in lateral and longitudinal directions. Any convenient angle 14 can be employed within the range of from about 20 to about A 45 angle is preferred.

Sheet 10 can be constructed ofany radiation-opaque material. Preferably, sheet 10 is constructed of thin, refractory metal, such as tantalum, columfbium, molybdenum and the like. Tantalum sheets about 0.003 in. thick have been used quite successfully. Prior art radiation shields required heavier and bulky sheets about 0.022 in. thick.

The improved radiation shield of the present invention can be fabricated by simpletechniques. As shown in FIG. 2, sheet 10a, containing diagonal slits as shown in FIG. 1 is wrapped around a form 14. This form could conveniently be a susceptor crucible of an induction furnace. Alternatively, form 14 could be removable once the radiation shield is fabricated so that the shield could be used on a separate susceptor. A slot 16 is maintained between the ends of sheet 10a in order to prevent the formation of a closed loop or electrical circuit throughout the entire length of sheet 10a. Slot 16 is conveniently about A in. Spacers 18, which can be dimples or other protrusions on the surface of the sheet 10a directed toward the form 14, space the sheet from the form. Sheet 10b is then wrapped around sheet 10a and the slot 16 in sheet 1011 is positioned about 60 to 120 from the slot 16 in sheet 10a. Spacers 18 of sheet 10b are employed between sheets 10a and 10b to prevent gross electrical shorting between adjacent sheets and also to maintain integrity of the plural layers. Preferably the jagged edges of slits 12 in sheet 10 are employed as spacers 18. The jagged edges of slits 12 are directed toward the form 14. In use, such jagged edges provide very small contact points of high electrical resistance between adjacent layers. Such contacts will burn olf or evaporate in use thus eliminating any low resistance short circuits. The remainder of the jagged edge acts as a together. Additional layers, such as 10c, 10d, 10e, and 107 are applied in a similar manner to form the completed radiation shield. The slots 16 in adjacent layers are offset to prevent any two slots from being in the same radial line with each other. The staggering of the slots 16 thus eliminates circular continuity and increases the effectiveness of the radiation shield. Preferably the radiation shield has about six layers. The layers can be held in place in any convenient manner such as by tying wires around the layers. In this form of the invention it is preferred that the slits in adjacent layers be angled in opposite directions, preferably at to each other. For example, sheet 10a may have slits at a 45 angle to the right, while sheet 10b may have slits at a 45 angle to the left.

An alternative form of the invention is shown in FIG. 3 wherein an elongated sheet 10 is spirally wrapped around the form or susceptor 14 employing spacers 18 between adjacent layers of sheet 10.

The improved radiation shields of the present inven tion are more compact than prior art shields and thus require less shielding space. This enables the induction coil to be positioned closer to the susceptor and thus provides higher coupling efiiciency between coil and susceptor. This radiation shield has mechanical flexibility due to the slits and thus thermal expansions and contractions resulting from use in an induction furnace do not create damaging warpa ge of the radiation shield. This shieldthus has a longer useful life than prior art devices. This shield is a distinct advance over the prior art since it has been used at temperatures as high as 2550 C. without damage. Prior art devices were severely damaged by warpa-ge at such temperature even if fabricated from the same metallic material.

Fabrication details of a typical radiation shield of the present invention are as follows. A tantalum sheet, about 0.003 to 0.0035 in. thick, is employed. This sheet is placed on a resilient surface, such as corrugated cardboard, and diagonal slits are cut into the sheet with a slitting tool having a fine cutting edge. The slits produced in this manner have jagged edges which subsequently aid in separating adjacent radiation shield layers. In producing a shield shown in FIG. 2, the lengths of the individual layer sheets are calculated as [follows wherein a A in. slot spacing is desired between ends of a given sheet when properly positioned in the shield.

Sheet: Length (inches) a 1r(D)-% 10b 1r(D+ A) A 10c 1r(D+ /2)% 10d 1r(D+%) A 10a 1r(D+1)- A 10f 1r(D+%)- Ar the number of the layer where X is the number of layers and D is defined above. In this case the single sheet is spirally wrapped around the form and wired in place. The shield of FIG. 3 can likewise be wrapped on a susceptor or wrapped on a removable form for subsequent use on a susceptor.

The spirally wound radiation shield of FIG. 3 wherein all the diagonal slits are in the same direction is not as effective as the shield of FIG. 2 wherein the slits in alternate layers are in opposing directions. This disadvantage can be overcome by changing the direction of the slits along the length of the spiral shield. For example, the length of the spiral shield corresponding to the first complete layer can have the slits at 45 to the right. The length of the spiral shield corresponding to the second complete layer can have the slits at 45 to the left. The portions of the spiral shield corresponding to layers three to six can have appropriate shifts in slit directions.

A radiation shield of the present invention consisting of six separate layers of 0.003 to 0.0035 in. thick tantalum sheet wherein each layer had multiple 45 angle slits with one-inch separation between slits and wherein the slits in adjacent layers were at 90 to each other and wherein each layer had a A-in. gap between the edges of circumferential ends had a total radial thickness of only A -in. This shield was employed in an induction furnace wherein the susceptor crucible temperature was about 2300-2400 C. The temperature at the outer surface of the shield was only about 900 C. demonstrating the effectiveness of the shield. This shield was used under these conditions for 120 consecutive heating runs wherein the operating time at temperature for each run was 30 min. There was no structural damage to the shield caused by thermal warpage during this time.

What is claimed is:

1. A radiation shield for use in a furnace around an inductively heated crucible and comprising at least one sheet of heat resistant, radiation opaque material arranged into an open-ended, cylindrical, multi-layered structure having a central open core of a diameter greater than the diameter of the crucible to be shielded, and having spacers positioned between adjacent layers to provide separation thereof, said sheets of radiation opaque material having formed therein a plurality of relatively closely-spaced, generally parallel slits arranged at an angle to the longitudinal axis of the cylindrical structure and located in a central portion of the sheet leaving an unslit edge portion on the top, bottom and ends of each sheet.

2. A radiation shield according to claim 1 wherein edge portions of at least some of the slits in the sheets project above the surface of the sheet providing spacers separating adjacent sheets, and wherein the slits in one layer are arranged in a staggered relationship to the slits in an adjoining layer.

3. A radiation shield according to claim 1 wherein the slits are formed in the sheets at angles of 45 to the longitudinal axis of the cylindrical structure.

4. -A radiation shield according to claim 2 wherein the heat resistant radiation opaque material is tantalum metal in the form of at least one thin sheet.

5. A radiation shield according to claim 1 wherein protuberances rising above the surface of the sheets form the spacers separating adjacent layers.

6. A radiation shield for use in a furnace around an inductively heated crucible and comprising a plurality of concentrically-arranged, open-ended, closely-spaced, cylindrical-like members, the innermost member having a diameter greater than the diameter of the crucible to be shielded and the surrounding members having successively increasing diameters, spacers positioned between adjacent members to provide separation thereof, each member comprising a sheet of heat resistant, radiation opaque material formed into an incomplete cylinder with the longitudinal edges of the sheet brought into proximate but separated relationship leaving a gap down the side of the member, said sheets each having formed therein a plurality of relatively closely-spaced, substantially parallel slits arranged at an angle to an edge of the sheet, said slits located inside the edges of the sheet leaving an unslit peripheral portion around the top, bottom and side edges of each sheet.

7. A radiation shield according to claim 6 wherein edge portions of at least some of the slits in the sheets project above the surface of the sheet providing spacers separating adjacent members.

8. A radiation shield according to claim 6 in which the slits in adjacent members are angled in opposite directions.

9. A radiation shield according to claim 6 wherein the heat resistant, radiation opaque material is tantalum.

10. A radiation shield according to claim 6 wherein the slits are formed at angles of 45 to the edges of the sheets.

11. A radiation shield for use in a furnace around an inductively heated crucible and comprising an elongated strip of heat resistant, radiation opaque mate-rial spirally wound into an open-ended, cylindrical structure having a height equal to the Width of the strip with an open core having a diameter greater than the diameter of the crucible to be shielded, and spacers located between and separating successive windings of the structure, said elongated strip having formed therein a plurality of relatively closely spaced slits, at least groups of which are arranged parallel to each other, said slits formed inside the top and bottom longitudinal edges of the strip to leave an unslit top and bottom edge portion running the length of the strip.

12. A radiation shield according to claim 11 wherein the slits terminate inside the end portions of the strip to leave an unslit edge portion at each end of the slit.

13. A radiation shield according to claim 11 wherein edge portions of at least some of the slits project above the surface of the winding providing-spacers separating successive windings.

14. A radiation shield according to claim 11 wherein the slits in some groups are angled in an opposite direction to the slits in other groups.

15. A radiation shield according to claim 11 wherein the heat resistant radiation opaque material is tantalum.

16. A radiation shield for use in a furnace wherein a radiation shield is arranged closely around a crucible with the radiation shield itself closely surrounded by an induction coil for inductively heating the crucible, said radiation shield comprising a plurality of concentricallyarranged, closely spaced, open-ended cylinder-like members, the innermost member having a central opening with a diameter only slightly greater than the outside diameter of the crucible to 'be shielded and the surrounding cylinder-like members having only slightly successively inc-reasing diameters to allow the nesting of the cylinders one within the next with only spacers located between adjacent cylinders, whereby said radiation shield has a small radical thickness all-owing it be arranged between a crucible and its heating induction coil with the induction coil close to the crucible for efficient heating thereof, said cylinder-like members each comprising a thin sheet of tantalum formed into an incomplete cylinder with the longitudinal edges of the sheet brought into proximate but separated relationship the member, said sheets plurality of substantially leaving a gap down the side of each having formed therein a parallel, diagonal slits, said slits 5 located inside the edges of the sheet leaving an unslit 0 member.

and side edges portions of the slits projecting of the sheet to provide the only sheets, the diagonal slits in each References Cited by the Examiner UNITED STATES PATENTS 12/1947 Sterick 263-50 X 6/1953 Schmer-tz 263-50 X 8/1956 Cryor 161-156 X 10/1959 Mercier 336-84 6/ 1965 Hagenbook 263-50 FOREIGN PATENTS 3/ 1959 Great Britain.

RALPH G. NILSON, Primary Examiner.

25 JAMES W.

LAWRENCE, Examiner.

Claims (1)

1. A RADIATION SHIELD FOR USE IN A FURNACE AROUND AN INDUCTIVELY HEATED CRUCIBLE AND COMPRISING AT LEAST ONE SHEET OF HEAT RESISTANT, RADIATION OPAQUE MATERIAL ARRANGED INTO AN OPEN-ENDED, CYLINDRICAL, MULTI-LAYERED STRUCTURE HAVING A CENTRAL OPEN CORE OF A DIAMETER GREATER THAN THE DIAMETER OF THE CRUCIBLE TO BE SHIELDED, AND HAVING SPACERS POSITIONED BETWEEN ADJACENT LAYERS TO PROVIDE SEPARATION THEREOF, SAID SHEETS OF RADIATION OPAQUE MATERIAL HAVING FORMED THEREIN A PLURALITY OF RELATIVELY

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